The present invention relates to a method of improving current efficiency of a cation-exchange membrane. More particularly, it relates to a method of improving current efficiency in an electrodialysis of an electrolyte by a cation-exchange membrane in an aqueous solution having hydroxyl group ions which comprises swelling a homogeneous cation-exchange membrane with a water-miscible organic solvent to reduce the permeability of hydroxyl group ions.
In general, cation-exchange memmbranes have high resistance to permeation of anions. However, they have relatively high permeability only to hydroxyl group ions in comparison with the other anions. This fact is well-known and is considered to be caused by high mobility of hydroxyl group ion in an aqueous solution in comparison with the other anions.
The phenomenon that the cation-exchange membranes have relatively high permeability to hydroxyl group ions, is sometimes effective, for example, to separate an alkali metal hydroxide by a diffusion dialysis using a cation-exchange membrane from an aqueous solution containing electrolytes of alkali metal hydroxide and a salt thereof.
However, on the other hand, the phenomenon is disadvantageous to reduce current efficiency by diffusion-permeation of hydroxyl group ions through the cation-exchange membrane in the electrical operation for separation or concentration of an aqueous solution of electrolytes containing hydroxyl group ions by a cation-exchange membrane. Accordingly, it is desired to use a cation-exchange membrane having low permeability of hydroxyl group ions for the purpose.